A few comments as ex montrealer

Old port is fine in winter. I would never go there in summer as too touristy and crowded, better in winter. Walkways aren't that bad, it's hills you want to be carefull of in winter.

The underground city is just a trashy mall with subway access

The hyatt place is not a great location

There are amazing places to eat breakfast in Montreal, I doubt the hyatt is one of them